My radiator fluid was flushed once due to having the fluid very dirty. It looks like it was rusted. The reason behind that was because in the summer I had pour water in it. Well I took in to a radiator place and had it flushed. Repalced the fluid with true Antifreeze fluid. This was done about 8 month ago. This morning i went to check my radiator fluid and its it totally dark brown rusted color again. Why is that. I did not pour water in there at all. It had pure antifreeze fluid. Isn't it suppose to be green?